1. Field of the Invention
The present invention relates to a semiconductor laser device used as a light source for optical information processing, a signal source for optical communications, or an excitation light source for fiber amplifiers.
2. Background Art
FIG. 13 shows the relationship between the wavelength and the optical output power of a conventional semiconductor laser device. The semiconductor laser device of FIG. 13 is constructed such that an SiO2 film is provided on the front end face (emitting end face) and a multilayer film made up of SiO2 films and amorphous silicon films is provided on the rear end face. In this case, the reflectance of the front end face is 6% while that of the rear end face is 94% (see, for example, “High-power visible GaAlAs lasers with self-aligned stripe buried heterostructure”, by T. Ohtoshi et al., Journal of Applied Physics (US), 1984, vol. 56, No. 9, pp. 2491–2496).
Referring to FIG. 13, as the optical output power changes from 1 mW to 30 mW, the emission wavelength changes from 780 nm to 786 nm. That is, the change in the wavelength per unit output power is 0.21 nm/mW. Therefore, the change in the wavelength per unit current is 0.21 nm/mA, assuming that the slope efficiency is 1 mW/mA.
Such a change in the wavelength is attributed to an increase in the temperature of the active layer due to an increase in the injection current. It is reported that the change in the wavelength per unit temperature is approximately 0.2–0.3 nm/° C. for AlGaAs semiconductor lasers and approximately 0.4–0.7 nm/° C. for InGaAsP semiconductor lasers (see, for example, “Optical Communications Device Engineering”, third edition, by Hiroo Yonezu, Kougakutosho Ltd., 1986, pp. 243–255).
The above conventional semiconductor laser device has a structure in which only an SiO2 film having a thickness of λ/4 is provided on the front end face (where λ is the wavelength). Therefore, the reflectance of the front end face is as high as 6%; the conventional semiconductor laser device cannot achieve low reflectance (1% or less).
Furthermore, the conventional semiconductor laser device exhibits a large change in the emission wavelength due to the temperature dependence of its gain, which causes a significant problem when it is used for applications requiring a stable-wavelength light source.